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Thank you!  To the point, I can get behind your idea that the slippery slope can be used in a valid way, not necessarily as an argument, but as a method of assessing a situation or state of affairs at a high level to try to understand the different possible sources of a specific outcome.

And it can be useful in coming to an understanding of how the principles need to be articulated in order to prevent this unintended outcome.  Sometimes the outcome can happen, even if it's not a consequence that follows logically, simply because there's a danger that many people will falsely extend it as slippery slope.

So, for example, ONLY by way of example, those who defend Pius XII's NFP against the charge will say that he only approved it for "grave" reasons.  But the problem happens where grave reasons can be extended and misinterpreted, that "graveness" admits of a certain amount of subjectivity ... and that this lack of precision in precisely delineating the criteria for the requisite "graveness" has led to the abuse of NFP.

It left out Reductio ad Hitlerum.

 ... the Jєωιѕн favorite.